Non-uniform Magnetic Field Distribution Research Articles

Inhomogeneous magnetic flux focusing in superconducting Josephson networks

The bending effects of a transverse magnetic field on planar superconducting structures in the Meissner state are investigated. Systematic measurements are performed on arrays made of four and six small Josephson junctions connected in parallel by superconducting films. By applying an external uniform magnetic field H, we measure the Josephson critical current ${I}_{c},$ which is sensitive to the local magnetic-field distribution, as a function of H. We find that the experimental data cannot be accounted for by the external magnetic field, and a strong focusing effect is present. Moreover, only by using a nonuniform magnetic-field distribution with a maximum in the center of the array are we able to fit the experimental data by numerical simulations. We show that, in the absence of self-field effects resulting from the Josephson currents, the nonuniform magnetic-field distribution is caused by the diamagnetic shielding produced by Meissner currents flowing in the superconducting electrodes. Three-dimensional finite element numerical calculations are performed on several planar geometries in a transverse external magnetic field to compute the magnetic-field distribution and the screening current patterns. In this way, we find a quantitative agreement with the observed inhomogeneous field distribution. The multiloop geometry has also been compared with equivalent networks, which considerably reduce the numerical calculations and substantially give the same inhomogeneous flux focusing.

Dynamic magnetic susceptibility of a superconductor with a relaxing magnetic moment

Within the framework of the diffusion approximation for the dynamic magnetic flux, a model is constructed for the dynamic magnetic susceptibility of a superconductor with a nonuniform and time-dependent magnetic field distribution in the sample. The possibility of appearance of peaks in the temperature (or frequency) dependence of χ″ in a structurally homogeneous superconducting sample in the presence of a magnetic-field-induced nonuniformity of the diffusion parameter is demonstrated. The character of the temporal evolution of the magnetic susceptibility during relaxation of the magnetic moment of the sample is predicted and its properties are investigated.

The spectral flattening of the low-energy component in gamma-ray bursts

Observations from BATSE show that the spectra of gamma-ray bursts are very wide, which cannot be described by a simple law. The distribution of the peak energy Ep [defined as the maximum of the Eγ2N(Eγ) spectrum], and of the spectral indices below and above Ep, is wide. In particular, the slope below Ep lies mainly between 0.5 and 1, which is inconsistent with the 4/3 predicted by the simple synchrotron radiation spectrum. Here we discuss the emission model for gamma-ray bursts based on the shock model proposed by Mészáros & Rees. We find that the synchrotron radiation of relativistic electrons accelerated by a strong shock, together with the finite lifetime of synchrotron radiation and the non-uniform magnetic field distribution, can explain the gamma-ray burst spectra, and the spectral index below the peak energy Ep can be flattened by the non-uniform magnetic field distribution; this flattening is insensitive to the specific form of the magnetic field variance. Furthermore, the peak energy Ep ∝ Γ4 is sensitively dependent on the bulk Lorentz factor.

Geminga's Soft X-Ray Emission and the Structure of Its Surface

We present a model to explain the decrease in the amplitude of the pulse profile with increasing energy observed in Geminga's soft X-ray surface thermal emission. We assume the presence of plates surrounded by a surface with very distinct physical properties: these two regions emit spectra of very distinct shapes which present a crossover, the warm plates emitting a softer spectrum than the colder surrounding surface. The strongly pulsed emission from the plates dominates at low energy while the surroundings emission dominates at high energy, producing naturally a strong decrease in the pulsed fraction. In our illustrative example the plates are assumed to be magnetized while the rest of the surface is field free. This plate structure may be seen as a schematic representation of a continuous but very nonuniform distribution of the surface magnetic field or as a quasi realistic structure induced by past tectonic activity on Geminga.

Boiling two-phase flows of magnetic fluid in a non-uniform magnetic field

The effects of a magnetic field on the characteristics of boiling two-phase pipe flow of a temperature-sensitive magnetic fluid are clarified in detail both theoretically and experimentally. Firstly, the governing equations of boiling two-phase flow of a magnetic fluid are presented and numerically solved. Secondly, the aspect of boiling two-phase flow is visualized with ultrasonic wave echo in the region of non-uniform magnetic field and various distributions of two-phase flow characteristics are obtained. Finally, the effect of a magnetic field on the stability of boiling two-phase pipe flow of a magnetic fluid under a non-uniform magnetic field is investigated both theoretically and experimentally. These fundamental studies show that the precise control and stabilization of boiling two-phase flow of a magnetic fluid are possible by using the magnetic force of the fluid effectively.

Analysis of Petschek-type reconnection in a Y-type magnetic field distribution

An analytical investigation is performed of Petschek-type reconnection in an incompressible plasma medium with a non-uniform magnetic field distribution, given by B = √ ( 2 z ¯ ) in complex variable notation. The initial two-dimensional (2D) configuration contains a current sheet, modelled as a tangential discontinuity, which runs along the positive x-axis in the complex plane, and which terminates in a Y-type neutral point at one end ( x = 0). In the model, reconnection is initiated through the introduction of a tangential electric field component in a localized part of the current sheet referred to as the diffusion region. Outside the diffusion region, which we approximate as an X-type neutral point, the behaviour of the magnetic field and plasma flow is governed by the equations of ideal magnetohydrodynamics (MHD), which for the purpose of this analysis are transformed into a Lagrangian coordinate system. With the restriction of a small reconnection rate, a perturbation method is applied to solve the MHD equations for the outflow region, containing the reconnected plasma, and the surrounding inflow region. The outflow region is treated as a thin boundary layer, and the usual matching procedure for problems of this type is used. In agreement with the results of previous studies of Petschektype reconnection in non-uniform, asymmetric magnetic field and velocity distributions, the boundary and interior structure of the outflow region is found to consist of a combination of Petschek-type shocks and tangential discontinuities. This is in contrast to the case of an initially uniform magnetic field distribution, analysed originally by Petschek, where the outflow region is entirely bounded by Petschek-type shocks and contains no interior discontinuities. The new features arise because the shocks propagate at the local Alfvén velocity in the medium, whereas plasma entering the outflow region is accelerated to the Alfvén velocity as measured at the reconnection site. As a result of the discrepancy between the plasma outflow speed and the shock propagation speed in the model configuration analysed here, the behaviour of the field and flow is qualitatively and quantitatively different on opposite sides of the X-line. In particular, the outflow region propagating into the direction of increasing field strength (i.e. x increasing) is bounded entirely by Petschek-type shocks, but contains a tangential discontinuity which extends the original current sheet through the leading point (or nose) into the interior of the outflow region. On the other hand, the outflow region propagating into the direction of decreasing field strength ( x decreasing, but remaining positive) is bounded on the front by tangential discontinuities, which emerge out of Petschek-type shocks, and merge into the original current sheet at the nose. When the reconnection rate drops to zero, there is an additional reverse type of behaviour which occurs in the trailing part; tangential discontinuities develop to form the tail end boundary of the outflow region in the former case, and in the latter case a tangential discontinuity develops which penetrates through the tail into the outflow region. We also investigate the consequences of a reconnection site which moves along the current sheet, analyse what happens when there are two successive pulses of reconnection, and briefly discuss the situation when the outflow region propagates beyond the edge of the current sheet.

Calculation method of linear induction motor supplied from symmetrical voltage source

A finite length of the primary part of a linear induction motor (LIM) contributes to nonuniform magnetic field distribution in an airgap. It causes the current asymmetry if the primary winding is supplied from a symmetrical voltage source. This paper presents a calculation method of the LIM performance. The method is based on Fourier series technique and takes into account the current asymmetry caused by the end effects. In order to illustrate an influence of current asymmetry the calculation results of electromechanical characteristics of a flat single-sided LIM are presented.

Effects of diffusion on magnetic excitations in films of yttrium iron garnet

We have investigated the effects of a diffusion located at the yttrium iron garnet-gadolinium gallium garnet (YIG-GdGaG) interface on spin-wave excitations. We calculate at ambient temperatures that a localized or surface spin-wave mode can be excited in chemical vapor deposited but not in liquid phase epitaxy (LPE) films of YIG. This localized state can be excited for the magnetic field applied parallel but not perpendicular to the film plane, in agreement with experimental observations. The dependence of spin-wave intensities on the direction of the magnetic field, observed in LPE films, is explained in terms of a nonuniform distribution of the cubic magnetic anisotropy field at the interface.